Electronic devices used in servers, CPUs (Central Processing Units) in personal computers, and the like are required to efficiently dissipate heat that is generated by semiconductor devices. Therefore, such an electronic device may have a structure having a heat spreader attached immediately above the semiconductor device that is made of a material having high thermal conductivity such as copper.
In this case, as the surfaces of a heat generation source and a heat spreader have fine concavities and convexities, a sufficient contact area cannot be obtained even when having the surfaces directly contact each other, hence the interface surface works as a great thermal resistance, which hinders efficient heat dissipation. Therefore, it is practiced in which a heat generation source and a heat spreader are connected with each other via a thermal interface material (TIM) where the objective is to reduce the contact thermal resistance.
To achieve the objective, such a thermal interface material is itself required to be a material having high thermal conductivity, and to have a characteristic that makes it possible to make contact with a large area with respect to the fine concavities and convexities on the surfaces of a heat generation source and a heat spreader.
Conventionally, a heat dissipation grease, a phase-change material (PCM), indium, or the like is used as such a thermal interface material. One of the major features of these materials used as a heat dissipation material is that it is possible with these materials to obtain a large contact area regardless of the fine concavities and convexities because these materials have high fluidity at an operating temperature limit of an electronic device or below.
However, such a heat dissipation grease or a phase-change material has low thermal conductivity of 1 W/m·K to 5 W/m·K. Also, indium is a rare metal whose price has been rising due to a great increase of demand related to ITO, hence inexpensive alternative materials have been anticipated.
With the background as such, a linearly-structured object made of carbon atoms represented by a carbon nanotube attracts attention as a heat dissipation material. A carbon nanotube is a material not only having a very high thermal conductivity (1500 W/m·K to 3000 W/m·K) in its axial direction, but also superior flexibility and heat resistance, which is considered to have a high potential as a heat dissipation material.
Thermally-conductive sheets using carbon nanotubes have been proposed including a thermally-conductive sheet having carbon nanotubes dispersed in resin, a thermally-conductive sheet having a bundle of carbon nanotubes contained with resin where the carbon nanotubes are made by oriented growth, and the like.